1. Field of the Invention
This invention relates to improved galvanic or sacrificial anodes that are useful in providing corrosion protection to submerged ferrous metal structures, e.g. offshore drilling platforms, and can be installed and replaced without the aid of a diver.
2. Background of the Art
It is well known that various metal structures used in corrosive environments such as salt water (ocean) must be provided with protection from corrosion. This protection may be provided by utilizing sacrificial anodes or impressed current anodes. Impressed current anodes are of two types, consumable and inert. The consumable anodes may be fabricated from scrap iron. The inert anodes are usually comprised of a platinum group material supported on titanium, niobium, or other film-forming metal. All impressed current anodes require a source of direct current to make the anode effective to provide cathodic protection to the metal structure. While impressed current anodes have some advantages, e.g. they can be driven at a high current output per anode, they also have disadvantages. They require on-board electric power or a power cable running from shore. The anodes are expensive to install and difficult to arrange so as to achieve good current distribution. Also, the anodes and interconnecting cables are vulnerable to damage.
Sacrificial anodes function by corroding in the marine environment in place of the metal structure. It is known that the cathodic protection provided by sacrificial anodes will last for a time dependent on the number and size of the anodes; when the sacrificial anodes have corroded to the end of their useful life, the sacrificial anodes must be replaced. The replacement of sacrificial anodes is a difficult task and usually requires divers to descend into the water and secure replacement sacrificial anodes to the metal structure. This is a very expensive procedure and divers cannot operate at depths much below about 300 feet.
Various workers in the field have attempted to provide sacrificial anodes which can be easily replaced and do not require a diver to enter the water for such replacement, for example, see U.S. Pat. No. 2,870,079 which describes a string of detachable sacrificial anodes linked to each other by a flexible cable. In use, the sacrificial anode string is oriented substantially horizontally and thus would not be effective in providing protection along the vertical of a support or leg of a metal structure used in marine environments. It is clear that a flexible string would not be as suitable as a rigid structure in that wave action or ocean currents can move the sacrificial anodes away from the metal structure that is to be protected, and it is known in the art that a certain spacing between the sacrificial anode and the structure to be protected must be maintained for adequate corrosion prevention. Furthermore, this string would not be easily secured in a precise location even by means of guides, in the absence of a diver, since by the flexible nature thereof, the wave action of the marine environment will move the string away from a precise location during installation.
U.S. Pat. No. 3,037,926 also teaches a method for providing cathodic protection on the horizontal for the interior surfaces of ships, i.e., tankers. The sacrificial anode is formed on a rod which is connected by a flexible cable to a ship. This reference does not teach that the string of sacrificial anodic material would be useful in providing galvanic protection to a submerged marine structure, and moreover, for the same reasons as given above for the flexible string described in U.S. Pat. No. 2,870,079, it would not be useful in providing corrosion protection to a marine structure, at a precise location, unless installed by a diver.
U.S. Pat. No. 4,298,445 teaches a method of providing cathodic protection by use of an impressed current anode. This reference discloses the use of a retractable anode; however, the anode disclosed therein is clearly taught as useful only as an impressed current anode. In view of the metal utilized in fabricating the anode of this reference (a platinum group metal coated on titanium or niobium), this anode would not be useful in providing cathodic protection unless a direct current was applied thereto to make such anode of the correct EMF to provide cathodic protection to the metal structure to which it was affixed.
U.S. Pat. No. 4,045,320 teaches a galvanic anode having an active material cast onto a metal support wherein said support comprises an expandable network of metal bound to at least one side of the anode or active material to hold said active material to the support metal. This reference is relevant in showing sacrificial anodes; however, as with the prior art, these anodes must be individually secured to the marine structure, by a diver.
U.S. Pat. No. 3,870,615 teaches a sacrificial anode having a one-piece pipe core of steel in a body of sacrificial anode material surrounding and supported by the central section of said pipe. This sacrificial anode is individually secured to the structure and thus would have to be replaced after use by a diver descending to the depth at which it is located and welding a replacement anode to the structure at that point.
U.S. Pat. No. 4,292,149 teaches an impressed current anode which includes an elongated electrode of platinized titanium or niobium helically wound around a rope. This reference provides a flexible impressed current anode. Moreover, it is clearly pointed out that a diver must be utilized to anchor the downwardmost portion of the anode to the structure which is being protected thereby.
U.S. Pat. No. 3,616,418 teaches an anode assembly for cathodic protection of submerged supporting legs of off-shore platforms. This anode comprises an elongated carrier cable which may be secured to the platform to span the space between the submerged legs thereof and carries an elongated anode along the length of the cable intermediate its ends. This reference also teaches an impressed current anode which is utilized in a horizontal mode. For the same reasons as discussed above for flexible systems of this sort, this particular anode structure would not be useful in providing vertical galvanic protection to a submerged marine structure.
U.S. Pat. No. 4,056,446 teaches a method to measure the cathodic protection along the height of a vertical platform sitting on the ocean floor. This reference teaches a cable apparatus which is utilized to move a reference electrode along the vertical to measure the cathodic protection at any point above the floor of the ocean. This reference is relevant in showing the concern with maintaining cathodic protection for structures in a marine environment; however, it does not teach the use of sacrificial anodes but is instead concerned with measurement of the protection achieved by any galvanic protection method.
U.S. Pat. Nos. 4,201,637 and 4,318,787 teach a method for replacing anodic material used to provide galvanic protection to a marine structure. In both patents a sacrificial anode type material either as pellets as a thixotropic suspension is passed through a tube into a container located in the vicinity at which cathodic protection is desired. In particular, it may be located where a cross member is secured to a vertical member of the marine structure. The necessary conductivity required to provide galvanic protection is lacking in this reference in that the pellets or dispersed material must be in contact to provide an electrically continuous material. Moreover, in this environment the typical sacrificial anode material such as zinc, aluminum, magnesium or alloys thereof would rapidly convert at the surface to the oxide which is much less reactive and would insulate the particles from each other and so not provide the required galvanic protection.
U.S. Pat. No. 4,309,263 teaches a clamp for securing sacrificial anodes to a marine placement. This clamp overcomes the problems with welding a preformed joint to the platform by providing adjustable clamping means. This reference still requires a diver to go into the depths for replacement of the sacrificial anode.
U.S. Pat. No. 3,992,272 relates to the problem of easily corrodible joints such a where a cross member or brace is joined to a vertical leg of a marine structure. The joint is coated with a concrete material so that corrosion protection in addition to cathodic protection (by an impressed current) is not as critical.
It is thus one object of the instant invention to provide a sacrificial anode that can be attached to a submerged ferrous metal structure without the intervention of divers.
It is another object of the instant invention to provide a corrosion protection system for submerged ferrous metal structure which comprises a plurality of individual, interchangeable units.
It is an object of this invention to provide a system for precisely locating a sacrificial anode in relationship to a portion of a submerged ferrous metal structure without the intervention of a diver.
Other objects and advantages of the instant invention will be apparent from a careful reading of the specification below.